Fuel cells are used to produce electricity when supplied with fuels containing hydrogen and an oxidant such as air. There are several different types of fuel cells known in the art; amongst these are solid oxide fuel cells (SOFC). SOFC are regarded as highly efficient electrical power generator that produces high power density with fuel flexibility. A typical SOFC includes a dense electrolyte sandwiched between a cathode layer and an anode layer. Air is passed over the surface of the cathode layer and a fuel containing hydrogen (H2) and carbon monoxide (CO) is passed over the surface of the anode layer opposite that of the cathode layer. Oxygen ions from the air migrate from the cathode layer through the dense electrolyte to the anode layer in which it reacts with the H2 and CO in the fuel, forming water and CO2, and thereby creating an electrical potential between the anode layer and the cathode layer of about 1 volt. To increase the voltage and power, multiple SOFC repeating units are stacked in electrical series forming a SOFC stack.
The CO and H2 fuel is provided to the SOFC stack in a form of a reformate stream. It is known in the art to use a fuel reformer such as a catalytic partial oxidation (CPOx) hydrogen reformer to process a hydrocarbon fuel such as methane to produce CO and H2. To improve stack power density and system efficiency, a portion of the effluent gas from the SOFC stack anode is recycled to the fuel reformer. The effluent gas is also known as the anode tail gas and includes a large amount of water vapor and CO2. When these gases are recycled back to the fuel reformer, endothermic steam reforming occurs in the fuel reformer, thereby lowering the temperature of the fuel reformer. To maintain the temperature of the fuel reformer at an elevated optimal level, a combustor and heat exchanger may be incorporated into the fuel reformer. A portion of the anode tail gas is supplied to the combustor and is combusted with depleted air from the cathode tail gas. The hot effluent from the combustor is then passed through the heat exchanger to add heat to the reforming process. It is also known in the art that certain fuels can be partially or fully reformed inside the SOFC stack anode, when the SOFC stack is at operating temperature. For example, fuels such as methane, methanol, ammonia, and dimethyl ether can be mixed with reformate and/or gas anode tail gas and introduced directly into the SOFC stack anode.
U.S. Pat. No. 5,451,249 discloses that a gas stream that emanates from a landfill may be treated to contain essentially a hydrocarbon such as methane, which can be used as a fuel source in a fuel cell power plant. The method taught by U.S. patent '249 includes passing the landfill gas through a hydrogen sulfide adsorption bed to reduce hydrogen sulfides to a level of 5 ppm or less; cooling the resultant gas to condense the water vapors and heavier hydrocarbons; adsorbing the water vapor and heavier hydrocarbon fractions to form a dried gas stream; removing the hydrocarbon contaminants through an activated carbon; and removing contaminants particulates from the dried gas stream. The method also produces a waste gas stream containing hydrocarbons and other contaminants with minimal BTU value; therefore requiring an additional fuel source to flare the waste gas stream into the atmosphere to prevent polluting the environment.
This method of treating landfill gas in order to obtain the purity of methane required for a fuel cell is complex, costly, and involves substantial losses in efficiency. It limits the economic attractiveness of using landfill gas as a fuel source for fuel cells due to the high level of treatments to remove substantially all impurities, which includes sulfur compounds that may poison the reformer catalyst and fuel cell.